=over

=item $SYSTEM_FD_MAX

=item $^F
X<$^F> X<$SYSTEM_FD_MAX>

The maximum system file descriptor, ordinarily 2.  System file
descriptors are passed to C<exec()>ed processes, while higher file
descriptors are not.  Also, during an
C<open()>, system file descriptors are
preserved even if the C<open()> fails (ordinary file descriptors are
closed before the C<open()> is attempted).  The close-on-exec
status of a file descriptor will be decided according to the value of
C<$^F> when the corresponding file, pipe, or socket was opened, not the
time of the C<exec()>.

=back